Histologic Studies of Ostracoderms, Placoderms and Fossil Elasmobranchs

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Abstract
InPlourdosteus. Pholidosteusand other brachythoracids, the gnathalia exhibit a limited number of dental fields with much the same distribution pattern. Each of these either carries semidentine teeth or marks the location of a column of semidentine (or, in cases, other dental hard tissue) which phyletically represents the product of fusion of originally independent teeth. The basal portion of the superognathalia and of the biting division of the inferognathal were regions of active growth where formation of new hard tissue went onpari passuwith the abrasion on the biting areas of the plates. The inferognathal formed during ontogeny by the fusion of two separate elements, a dental component on which all the dental fields of the plate were situated, and an axial component which was devoid of such fields. InHolonema, the narrow ridges on the lingual side of the gnathal plate of the lower jaw consist in part of semidentine; even these formed phyletically by the fusion of teeth. Dermal jaw‐elements with quite the same developmental and structural features as those of brachythoracid or holonematid arthrodires are unknown in other fishes, including elasmobranchs.